An image sensor circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge. In a conventional four transistor CMOS imager, the active elements of a pixel cell perform: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. In a three transistor pixel cell the active elements of a pixel cell perform: (1) photon to charge conversion; (2) accumulation of image charge by the photoconversion device; (3) resetting the photoconversion device to a known state before charge accumulation; (4) selection of a pixel for readout; and (5) output and amplification of a signal representing the pixel charge.
Referring to FIGS. 1A and 1B, a semiconductor wafer fragment of a conventional CMOS image sensor four-transistor (4T) pixel 10 is shown. A view of a section of the conventional pixel 10 taken along line 1B-1B of FIG. 1A is shown in FIG. 1B. The pixel 10 comprises a transfer gate 50 for transferring photoelectric charges generated in a pinned photodiode 21 to a floating diffusion region 25 acting as a sensing node. The floating diffusion region 25 is connected to a reset transistor having a reset gate 40 for resetting the region 25 with a drain terminal connected to a supply voltage, e.g. Vaapix. A source follower transistor having a gate 60 connected to region 25 and a row select transistor having a gate 80 are also included in the pixel 10. The pixel output is at the source of the row select transistor. Impurity doped source/drain regions 22 are provided about gates 40, 60, 80. Spacers 92 may be formed along the sides of gates 40, 50, 60, 80.
As shown in FIG. 1B, the photodiode 21 is illustratively a shallow pinned photodiode just beneath the surface 15 of substrate 20. The pinned photodiode 21 typically has a photosensitive p-n-p junction region comprising a p-type surface region 24 and an n-type photodiode region 26 within the p-type substrate 20. Trench isolation regions 28 are formed in the substrate 20 to isolate the pixel 10 from other pixels within a pixel array. Contacts 32 (FIG. 1A) may be formed in an insulating layer (not shown) to provide an electrical connection to the source/drain regions 22, floating diffusion region 25, and various transistor gates.
The performance of conventional image sensors, including CMOS image sensors, is often limited by a low signal-to-noise ratio (SNR) of the pixels, where SNR is a measure of the relative magnitude of a signal compared to the uncertainty or noise in the signal for each pixel. Low SNR reduces image quality and may be caused, for example, by high fixed pattern noise (FPN), and other factors such as poor sensitivity and high dark current. One common source of noise is pixel reset noise (or kT/C noise), which is often attributed to thermal noise uncertainty associated with a pixel reset level. In an imager, KT/C noise is the dominant source of temporal noise at low light levels.
Because a pixel signal Vsig is measured relative to its reset level Vrst, high kT/C noise can interfere with the signal output from the pixel. Correlated double sampling (CDS) techniques and other techniques have been used in attempts to reduce pixel-level noise, including fixed pattern noise, and to increase the signal-to-noise ratio for integrating image sensors. However, conventional image sensor circuits are usually not capable of effectively reducing noise over a range of signal intensities such as, for example, different levels of light intensity. At high light levels, CDS techniques usually cannot reduce unwanted pixel noise in conventional image sensors.
Conventional image sensors are often not adequate for reducing all types of noise generated during image acquisition. At mid-high light levels, photon shot noise becomes the dominant noise source. Photon shot noise is often greater than kT/C noise associated with pixel reset. Reduction of photon shot noise requires averaging which requires multiple frames. In addition to the need to reduce noise, and because pixels are continually being scaled down in size, there is an increasing need to effectively reduce or eliminate noise to achieve a higher overall SNR, and to improve image quality.
It is therefore desirable to improve the design of pixel circuitry to effectively enhance SNR over a range of signal intensities and to improve global shutter operation. Such improved circuit design will yield improved output response even when pixel sizes are scaled down.